Patch "ext4: don't lock buffer in ext4_commit_super if holding spinlock" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

ext4: don't lock buffer in ext4_commit_super if holding spinlock

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
ext4-don-t-lock-buffer-in-ext4_commit_super-if-holding-spinlock.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Theodore Ts'o <[email protected]>
Date: Sun, 13 Nov 2016 22:02:29 -0500
Subject: ext4: don't lock buffer in ext4_commit_super if holding spinlock

From: Theodore Ts'o <[email protected]>

commit 1566a48aaa10c6bb29b9a69dd8279f9a4fc41e35 upstream.

If there is an error reported in mballoc via ext4_grp_locked_error(), the code is holding a spinlock, so ext4_commit_super() must not try to lock the buffer head, or else it will trigger a BUG:

  BUG: sleeping function called from invalid context at ./include/linux/buffer_head.h:358
  in_atomic(): 1, irqs_disabled(): 0, pid: 993, name: mount
  CPU: 0 PID: 993 Comm: mount Not tainted 4.9.0-rc1-clouder1 #62
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.8.1-0-g4adadbd-20150316_085822-nilsson.home.kraxel.org 04/01/2014
   ffff880006423548 ffffffff81318c89 ffffffff819ecdd0 0000000000000166
   ffff880006423558 ffffffff810810b0 ffff880006423580 ffffffff81081153
   ffff880006e5a1a0 ffff88000690e400 0000000000000000 ffff8800064235c0
  Call Trace:
    [<ffffffff81318c89>] dump_stack+0x67/0x9e
    [<ffffffff810810b0>] ___might_sleep+0xf0/0x140
    [<ffffffff81081153>] __might_sleep+0x53/0xb0
    [<ffffffff8126c1dc>] ext4_commit_super+0x19c/0x290
    [<ffffffff8126e61a>] __ext4_grp_locked_error+0x14a/0x230
    [<ffffffff81081153>] ? __might_sleep+0x53/0xb0
    [<ffffffff812822be>] ext4_mb_generate_buddy+0x1de/0x320

Since ext4_grp_locked_error() calls ext4_commit_super with sync == 0 (and it is the only caller which does so), avoid locking and unlocking the buffer in this case.

This can result in races with ext4_commit_super() if there are other problems (which is what commit 4743f83990614 was trying to address), but a Warning is better than BUG.

---
 fs/ext4/super.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

@@ -4550,7 +4550,8 @@ static int ext4_commit_super(struct supe
 				&EXT4_SB(sb)->s_freeinodes_counter));
 	BUFFER_TRACE(sbh, "marking dirty");
 	ext4_superblock_csum_set(sb);
-	lock_buffer(sbh);
+	if (sync)
+		lock_buffer(sbh);
 	if (buffer_write_io_error(sbh)) {
 		/*
 		 * Oh, dear.  A previous attempt to write the

@@ -4566,8 +4567,8 @@ static int ext4_commit_super(struct supe
 		set_buffer_uptodate(sbh);
 	}
 	mark_buffer_dirty(sbh);
-	unlock_buffer(sbh);
 	if (sync) {
+		unlock_buffer(sbh);
 		error = __sync_dirty_buffer(sbh,
 			test_opt(sb, BARRIER) ? WRITE_FUA : WRITE_SYNC);
 		if (error)

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/ext4-reject-inodes-with-negative-size.patch queue-4.9/ext4-fix-in-superblock-mount-options-processing.patch queue-4.9/ext4-don-t-lock-buffer-in-ext4_commit_super-if-holding-spinlock.patch queue-4.9/ext4-fix-mballoc-breakage-with-64k-block-size.patch queue-4.9/ext4-do-not-perform-data-journaling-when-data-is-encrypted.patch queue-4.9/ext4-use-more-strict-checks-for-inodes_per_block-on-mount.patch queue-4.9/ext4-add-sanity-checking-to-count_overhead.patch queue-4.9/ext4-return-enomem-instead-of-success.patch queue-4.9/ext4-fix-stack-memory-corruption-with-64k-block-size.patch